The field of the invention is systems and methods for magnetic resonance imaging (“MRI”) and nuclear magnetic resonance (“NMR”). More particularly, the invention relates to systems and methods for designing parallel transmission (“pTx”) radio frequency (“RF”) pulses for use in saturation of inhomogeneous broad lines in MRI and NMR.
There are many applications of MRI and NMR that require saturation of broad inhomogeneous lines. For example, in NMR “hole burning” experiments are commonly used to measure various properties of liquid and solid samples such as relaxation and molecular motion. In MRI, similar experiments are used, however for very different purpose. An interesting class of MRI experiments that involve saturation of broad inhomogeneous lines is magnetization transfer (MT). In the following we describe in more details the application of the present invention to MT MRI because it is an important application of the present invention and is an important class of experiments that can be used to image a variety of clinically important conditions. However, the invention is not limited to MT imaging using MRI. The invention is applicable to any application that requires saturation of broad inhomogeneous lines, MT imaging and saturation is simply a special case.
In the brain, a significant fraction of the spins in the bound pool are attached to myelin molecules that cover the axon of neurons. Therefore, MT pulses can be used to obtain indirect information about the myelin content of the brain, which is a crucial metric for assessing and staging white matter diseases, such as multiple sclerosis. MT pulses are also used in a recently developed form of spectroscopic imaging developed called chemical exchange saturation transfer (“CEST”). CEST is being investigated by dozens of research groups around the world and has shown promise for the in vivo quantification of pH, ATP concentration, and lactic acid concentration. Thus, CEST has applications for stroke, cancer, and imaging of gene expression via the reporter gene technique. Another clinical application of MT pulses is using them for background suppression techniques. For example, MT-based background suppression is widely used in time-of-flight angiography, which is one of the main applications of clinical MRI.
A significant limitation of saturation pulses, such as those used in MT MRI, is their large power consumption. In MT MRI, the saturation transfer effect is a relatively small effect (i.e., only a small fraction of the energy stored in the bound pool is transferred to the free water pool), so that large amounts of energy need to be “dumped” into the bound pool to detect significant changes in the magnetic resonance image. As a result, MT pulses need to be long and need to have a high amplitude. At ultra-high magnetic fields (e.g., greater than 3 Tesla), the duration and amplitude of these MT pulses cause a major problem of dramatically increasing the specific absorption rate (“SAR”). In hole burning experiments, where the goal is to saturate spectral frequency regions of a broad inhomogeneous lineshape, large saturation amplitudes need to be used as well, which result in large energy deposition in the sample or patient (SAR). SAR is a measure of the energy deposited in the patient by the MRI procedure and is limited by the Food and Drug Administration (“FDA”) in the United States. These power limits are too restrictive for MT pulses to be used at ultra-high magnetic fields.
Thus, there remains a need to design MT and saturation pulses that have lower power requirements, such that the MT and saturation pulses can be used at ultra-high magnetic fields without exceeding regulatory or patient safety limits on SAR.